We are running tomcat application server that handle over 100 concurrent sessions. In the last 2 months the most active users noticed that sometimes they are getting kicked
You can search Tomcat's bug database, but it would be better to have another look at your web application first. The chances that there is something wrong with Tomcat are very low.
Try to investigate what causes session invalidation. Are you using filters? Do you have cross-context requests? Try adding logging information for every request to find out, when exactly the session is lost.